ublk: fix command op code check

In case of CONFIG_BLKDEV_UBLK_LEGACY_OPCODES, type of cmd opcode could
be 0 or 'u'; and type can only be 'u' if CONFIG_BLKDEV_UBLK_LEGACY_OPCODES
isn't set.

So fix the wrong check.

Fixes: 2d786e66

 ("block: ublk: switch to ioctl command encoding")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230505153142.1258336-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>